1. Field of the Invention
The present invention relates to an optical glass, a preform for press-molding, a method for the preparation of a preform for press-molding, an optical part and a process for the production of an optical part. More specifically, the present invention relates to an optical glass having a B2O3—SiO2—La2O3—Gd2O3—ZnO—Li2O composition, having a viscosity property with which a preform for press-molding is hot-shapable, having optical constants such as a refractive index (nd) of 1.75 to 1.85 and an Abbe's number (νd) in the range of from 40 to 55, having a low transition temperature imparted and having suitability to mold-shaping, a press-molding preform made of the above optical glass; a method for efficiently preparing the above preform; an optical part made of the above optical glass; and a process for producing an optical part from the above press-molding preform.
2. Prior Art of the Invention
In recent years digital cameras have come to be popular, and the integration degree and functions of machines and equipment used for optical systems are rapidly increased. Under the circumstances, it is increasingly demanded to attain the higher accuracy of optical systems and decrease the optical systems in weight and size. For materializing such demands, optical designing using an aspherical lens is coming to constitute a main stream. For stably supplying a large amount of spherical lenses made of high-function glass at a low cost, therefore, attention is paid to a molding technique for directly forming an optical surface by press-molding without any cutting and polishing steps. Demands for an optical glass having high-functions (e.g., high refractivity—low dispersion; high refractivity—high dispersion) and having suitability to mold-shaping are increasing year after year.
Conventionally, glasses containing B2O3, La2O3, etc., as essential components are known as an optical glass having optical constants of a high refractive index (nd≧1.75) and low-dispersion (νd≧40). In these glasses, however, priority has been given to improvements in chemical durability, devitrification resistance against heat and hot shapability, and the high-temperature moldability required for hot press-molding of a glass gob preform has not been fully taken account of. Particularly, no high-refractivity low-dispersion optical glass having a refractive index of nd≧1.75 and an Abbe's number of νd≧45 is suitable for molding a gob preform, which is an obstacle for mass-producing aspherical lenses by precision press-molding. The reason therefor is as follows. Since the above glass generally has a high liquidus temperature or a low viscosity at high temperatures, the high-temperature moldability of a gob preform is deteriorated, and it is difficult to prepare gob preform having high accuracy.
Precision press-molding of a glass is a method in which a glass preform (gob preform) is molded under pressure at a high temperature with a mold having a cavity having a predetermined form, to obtain a glass molded article having a form of an end product or a form very close thereto. According to the precision press molding, molded articles having a desired form can be mass-produced at high productivity. For this reason, various optical glass products such as a spherical lens, an aspherical lens, a diffraction grating, etc., are produced by precision press-molding these days. For obtaining an optical glass product by precision press-molding, naturally, it is required to press-mold a glass preform at a high temperature as described above, so that a mold for the pressing is exposed to high temperatures and that high pressures are applied thereto. From the viewpoint of suppressing damage caused on a mold itself and a mold release film provided on an inner surface of the mold, under an high-temperature environment for the press-molding, therefore, it is desired to decrease the glass transition temperature (Tg) and the sag temperature (Ts) of a gob preform for glass molding. When a glass preform for precision press-molding is prepared by hot shaping, a glass having a high liquidus temperature (LT) is liable to devitrify, and such a glass is poor for mass-productivity, so that it is desirable to decrease the LT of the glass. For preparing precision-press-molding preforms accurately and uniformly by hot shaping, further, the high-temperature viscosity of a glass at its liquidus temperature is required to be at least 6 dPa·s.
Conventional optical glass, particularly, an optical glass for precision press-molding, has a liquidus temperature of far greater than 1,000° C. when it has a refractive index (nd)>1.75 and an Abbe's number (νd)>40. Such a glass is poor in thermal stability and is liable to devitrify when a gob preform is molded. Such a glass is therefore poor in mass-productivity, and it is difficult to prepare glass gob preform for press-molding, therefrom accurately and uniformly.